The mechanisms responsible for the alteration of chemical carcinogenesis by the dietary lipotropes, choline, methionine, folic acid and vitamin B-12 have been studied. The metabolism and carcinogenic activity of ethionine in different species is being compared. Correlations between the tissue levels of S-adenosylmethionine, S-adenosylhomocysteine, and 5-methylcytosine in animals treated with carcinogens, liver tumor promoters and methyl-deficient diets are being determined. The effects of methionine antagonists, and of dietary methionine and choline on carcinogenesis in liver and other organs are under investigation using standard bioassays. The effects of methylase inhibitors on the methylation of macromolecules and on carcinogenesis in vivo are determined.